To provide optimum coupling efficiency and power transfer in the field of solid state power generation and amplification using semiconductor diodes, such as Gunn diodes and IMPATT diodes, the relatively low impedance of these semiconductor devices should be closely matched to the relatively high impedance of the wave propagating structure to which the diode is coupled. This problem becomes particularly significant as the power requirements (and thus area) of these solid state devices is increased, thereby decreasing the device impedance and increasing the device capacitance.
One common prior art technique known to us for approaching this impedance matching problem involves the use of a so called quarter wave (.lambda./4) impedance transformer, which is typically a solid metal cylinder having its longitudinal axis equal to approximately one quarter of the wavelength or wavelength range of interest. One end of this cylinder is bonded for ohmic contact to a semiconductor diode package, and the other end of the metal cylinder is joined to an inner conductor of a relatively high impedance coaxial line used for coupling power from the diode package to some external circuit or cavity, such as a power combining cavity. Examples of such impedance transformers are disclosed in U.S. Pat. Nos. 3,842,370 and 3,931,587, assigned to the present assignee.
The above type of impedance transforming arrangement presents several structural disadvantages which tend to reduce the coupling efficiency of circuits using the arrangement, particularly when relatively large area and thus low impedance diodes are required. First of all, since ohmic contact bonding is made between one surface of the relatively small diode package and the larger cylindrical surface of the above described quarter-wave impedance transformer, the resulting ohmic contact between these members is frequently nonuniform. Such contact or contacts frequently occur only at localized regions on the surfaces of these two abutting structures, and thereby produce high localized currents and corresponding high I.sup.2 R power losses at the diode package/impedance transformer interface. It is simply difficult to make good uniform ohmic contacts using this prior art approach of pressure bonding the diode package between the quarter-wave impedance transformer's end surface and a supporting heat sink for the diode which typically couples the outer coaxial conductor of the coupling structure.
Another problem which is encountered when the area of the semiconductor diodes is increased to increase their power generation capability relates to the effect that the corresponding increase in diode capacitance has on the diode package resonance. That is, as the capacitance of the diode is increased in direct proportion to its increase in area, it remains no longer resonant with the inductance of the package housing and electrical leads for the diode. This means that the total package inductance must be changed accordingly in order for it to remain substantially series resonant with the increased capacitance of the diode, an obviously desirable characteristic for achieving maximum power transfer between the diode and the external circuit to which it is coupled.
A further disadvantage of this prior art quarter-wave impedance transformer approach to impedance matching between the diode and its external circuitry resides in the relatively abrupt step transformation between the low impedance of the diode and the relatively high impedance of the coaxial circuit which is only one quarter wavelength from the diode. This abrupt impedance transition is simply not consistent with good broadband circuit operation, and it therefore limits the achievable operational bandwidth of the circuit structure.